What is "blautopf"

The Blautopf (German for Blue Pot; "blau" means blue, "Topf" means pot) is a spring that serves as the source of the river Blau in the karst landscape on the Swabian Jura's southern edge, in Southern Germany. It is located in the city of Blaubeuren, approximately west of Ulm. It forms the drain for the Blau cave system and feeds the river Blau, which after , flows into the river Danube in the city of Ulm. Because of its high water pressure, the spring has developed a funnel-like shape, which at its deepest point has a depth of 21 metres (69 ft). The water's peculiarly blue color, varying in intensity due to weather and flow, is the result of physical properties of the nanoscale limestone densely distributed in the water. The particles are so small that the so-called Rayleigh scattering of light takes place, preferentially scattering the blue color in the visible light. Similar effect is observed at the Blue Lagoon in Reykjavik, Iceland, where the blue color originates from nanoscale silica particles.